Compounds shown to be effective in the treatment of cancer cells typically affect such cells by inducing maturation (i.e. slowing growth) of the cells or by killing the cells (i.e. necrosis), because the compound itself is toxic. Compounds which slow cancer cell growth or are toxic to the cancer cells are often disadvantageous because the compounds themselves often adversely affect normal cells.
It has been discovered that cancer cells can be induced to kill themselves (i.e. to undergo programmed cell death, hereinafter referred to as "apoptosis"). Accordingly, compounds which can induce cancer cells to kill themselves are less likely to adversely affect the patient because the compound affects normal cells to significantly less of a degree than cancer cells (i.e. normal cells are able to recover at doses which are effective for the treatment of cancer cells).
More specifically, the process of necrosis is characterized by the inflammation of a colony of cells which include both cancer and normal cells. When cells are contacted with a necrosis-inducing agent, the cells breakdown into relatively large fragments with DNA typically withstanding any significant fragmentation (i.e. DNA being typically greater than 100,000 bases). Thus, both cancer cells and normal cells are affected.
The mechanism of apoptosis is not clearly understood. It is believed that apoptosis arises due to a change in the gene expression in the cell causing the cell to program and induce its own death. The result is a breakup of the genetic messenger, DNA, into smaller enveloped components which can be absorbed by adjacent cells without harmful effect.
Apoptosis is characterized by the selective programmed destruction of cancer cells into relatively small fragments with DNA becoming highly fragmented (i.e. the resulting fragments typically have no more than about 200 bases.) During apoptosis, cell shrinkage and internucleasomal DNA cleavage occurs, followed by the fragmentation of the DNA. Eventually the cell disintegrates into small fragments.
Thus, there is a significant difference in the results achieved by necrosis as compared with apoptosis. The cellular material remaining after necrosis is large and relatively difficult for unaffected cells to assimilate. In the aftermath of apoptosis, because the remaining material is in relatively small units, they are readily assimilated by unaffected cells.
Thus apoptosis-inducing agents possess significant advantages over compounds which induce necrosis. Such agents are not only selective for cancer cell destruction, but also enable the fragmented cellular material to be safely assimilated by the body.
Benzamide riboside has been shown to be a compound capable of inducing differentiation of cancer cells. More specifically, benzamide riboside has been shown to be cytotoxic to S49.1 lymphoma cells by Karsten Krohn et al.,.J. Med. Chem., Vol. 35, pp 511-517 (1992) and to human myelogenous leukemia cells by Hiremagalur N. Jayaram et al., Biochem. Biophys. Res. Commun., Vol. 186, No. 3, pp. 1600-1606 (1992), each of which is incorporated herein by reference.
As indicated in H. N. Jayaram et al., benzamide riboside inhibits the enzyme inosine 5'-monophosphate dehydrogenase (IMP dehydrogenase) which is necessary for cell growth. However, in vitro inhibition of IMP dehydrogenase requires very high concentrations of benzamide riboside, suggesting that the compound may require conversion to a different form to exert IMP dehydrogenase inhibitory activity. Accordingly, benzamide riboside has been described as a prodrug.
More recently, Kamran Gharehbaghi et al. Int. J. Cancer, Vol. 56, pp. 892-899 (1994) disclosed that benzamide riboside exhibited significant cytotoxicity against a variety of human tumor cells in culture through a derivative of benzamide riboside, benzamide adenine dinucleotide (BAD).
The references discussed above show that benzamide riboside acts through its dinucleotide derivative. While an inhibition of cell growth was observed, there was no reported observation of apoptosis. This is because human myelogenous leukemia K562 cells used for these studies possess a genetic makeup which is strongly resistant to apoptosis. Therefore, the work done to date on benzamide riboside has focused on inhibition of cell growth through reduction of IMP dehydrogenase and not to a method of inducing cells to undergo apoptosis, which is clearly an advantageous process for cancer cell destruction.